1. Field of the Invention
The present invention relates to deicing of aircraft positioned on a taxiway just prior to takeoff. More particularly, the present invention is directed to the application of infrared radiation to an aircraft for rapidly deicing and substantially drying an aircraft immediately prior to takeoff.
2. Background Art
Federal Aviation Regulations (FAR) prohibit the takeoff of an aircraft when snow, ice, or frost is adhering to wings, propellers, control surfaces, engine inlets, or other critical surfaces of the aircraft. These regulations are a result of the detrimental influence that foreign matter on the exterior surface of an aircraft has on the control and safety of the aircraft during takeoff and flight.
Aircraft typically have a smooth exterior surface so as to produce a smooth or laminar air flow over the exterior or aerodynamic surfaces of the aircraft during takeoff and flight. A laminar air flow minimizes drag, maximizes lift, and optimizes control of the aircraft. The adhering of frozen contaminates, such as ice and dirt, to the exterior surface of an aircraft increases the surface roughness on the aerodynamic surfaces. Increased surface roughness alters the laminate flow of air to a turbulent flow which negatively affects drag, lift, and control on the aircraft.
Ice, snow, or frost often forms on the aircraft to a thickness and surface roughness similar to medium or coarse sandpaper. When so formed on the leading edge and/or upper surface of a wing it can reduce wing lift by as much as 30 percent and increase drag by 40 percent. These changes in lift and drag significantly increase stall speed, reduce controllability, and alter aircraft flight characteristics. In the winter when an aircraft is left outside overnight, ice can build up on the wings several inches thick. Thicker and/or rougher frozen contaminants can have a greater negative affect on lift, drag, stall speed, stability, and control.
These adverse effects on the aerodynamic properties of an aircraft may result in sudden deviation of the aircraft front its intended flight path. Such deviations may not be preceded by any indication or aerodynamic warning to the pilot. Accordingly, it is imperative that takeoff not be attempted until, as required by regulation, all critical surfaces of the aircraft are free of adhering ice, snow, or frost formations. Once the aircraft has taken off, the heated exhaust or bleed air from the engines can be circulated through ducts in the aircraft to warm the critical surfaces and prevent their reicing.
The required removal of frozen contaminants from an aircraft is referred to as the "clean aircraft concept." The common practice for obtaining a clean aircraft prior to takeoff has two steps. First, deicing is accomplished by applying heated aqueous solutions of Freezing Point Depressant (FPD) fluids to the surface of the aircraft. The theory of applying FPD fluids is to decrease the freezing point of water in its liquid or frozen state, thereby causing at least a portion of the ice to melt so that the ice slides off the aircraft. Second, if desired, anti-icing or the preventing of reicing is accomplished by applying SAE or ISO Type II fluids (hereinafter "anti-icing fluids") to the cleaned surface of the aircraft. Anti-icing fluids are effective anti-icers because of their high viscosity and pseudoplastic behavior. The anti-icing fluids are designed to remain on the wings of an aircraft during ground operations, thereby preventing the formation of ice on the surface of the aircraft.
Several different types of FPD fluids have been developed during the past 40 years and many are in common use today. Each of these various fluids has unique characteristics and handling requirements. The FPD fluids are applied to critical aircraft surfaces, i.e., wings, tail section, and fuselage, through conventional spraying mechanisms. If the ice persists, or recrystallizes on the aircraft, application of FPD fluid is repeated until a clean aircraft is obtained. Although this method of deicing is functional, there are numerous problems and drawbacks associated with it.
One of the problems associated with the use of FPD and anti-icing fluids is the delay resulting from refreezing. Deicing and anti-icing fluids are typically applied to the aircraft at the terminal prior to or shortly after loading of passengers. Although the fluids are helpful in preventing the reicing of aircraft, the combination of freezing weather conditions traditionally associated with the use of the fluids, the extended period of time between the application of the fluids and takeoff, and the gravitational runoff of the fluid coating often results in reicing of the aircraft as the aircraft waits on the taxiing runway in preparation for takeoff.
As a result of this reicing, the aircraft is often required to return to the terminal where the deicing fluid is again applied to the aircraft and the process begins again. This delay of a single aircraft, however, creates a chain reaction which not only delays surrounding aircraft but also connecting flights. This delay creates a burden for both passengers and scheduling.
The actual spraying of the deicing fluids can also be detrimental to the aircraft. Although the deicing fluids should be applied to critical surfaces of the aircraft relating to lift, it is important that the deicing fluids not pool in balance bays, control cavities, and gap fields or be directly sprayed into sensor orifices and probes along the fuselage of the aircraft. The application or pooling of the deicing fluids in such areas can result in a number of problems, including for example, inaccurate reading of aircraft instruments, freezing the movement of control surfaces, and fracturing of seals. Improper spraying can also damage protruding equipment from the aircraft such as antennas. Finally, due to their high viscosity, anti-icing fluids should not be applied to areas such as pitot heads, angle-of-attack sensors, control surface cavities, cockpit windows, nose of fuselage, lower side of randome underneath nose, static ports, air inlets, and engines. The application of anti-icing fluids to such areas can obscure the vision of the pilot and produce inaccurate readings of aircraft instruments.
Even if the deicing fluid is properly applied to the aircraft, there are negative side effects resulting from the deicing fluid adhering to the aircraft. Based on wind, temperature, and elevation conditions at a runway having a defined length, all aircraft have a critical weight which they cannot exceed for the given runway. If an aircraft exceeds the critical weight, it will be unable to obtain lift-off in the limited length of runway. This concept is referred to as the "balanced field." As the deicing fluid is applied to the aircraft, the weight of the aircraft increases. To compensate for this added weight, the aircraft is required to either limit its fuel, thereby limiting the duration it can remain aloft, or limit the number of passengers it takes, thereby decreasing its profit margin. Both of these choices negatively affect the airline industry.
Furthermore, as with the existence of ice, the existence of a deicing fluid on the exterior surface of the aircraft also reduces the lift and increases the drag on the aircraft. The reduction in lift requires the aircraft to further limit its weight in order to obtain lift-off. Although many aircraft are capable of reaching speeds during takeoff which substantially remove the deicing fluid from the aircraft, other smaller aircraft are not capable of reaching such speeds. Such aircraft are required to use lighter and less effective deicing fluids, thereby increasing the risk of reicing.
Deicing fluids can also be very expensive to apply, especially if the deicing fluid has to be applied more than once to prevent refreezing. Furthermore, deicing fluids are a potential health hazard to those who are exposed to the fluids. Commercially available FPD fluids used for aircraft deicing are ethylene, diethylene, and propylene glycol. Ethylene and diethylene glycol are moderately toxic to humans. Swallowing small amounts of ethylene and diethylene glycol may cause abdominal discomfort, pain, dizziness, and affects on the central nervous system and kidneys. Exposure to vapors or aerosols of any FPD fluid may cause transitory irritation of the eyes. Exposure to ethylene glycol vapor in a poorly ventilated area may cause nose and throat irritations, headaches, nausea, vomiting and dizziness.
Furthermore, all glycols cause some irritation upon contact with the eyes or the skin. Although the irritation is described as "negligible, " chemical manufacturers recommend avoiding skin contact with FPD fluids and wearing protective clothing when performing normal deicing operations. Accordingly, not only are traditional deicing fluids dangerous to those who apply them and work around them, they are also potentially hazardous to passengers who could be exposed to vapors from the deicing fluids.
Finally, in light of the hazardous nature of the deicing fluids and the vast quantities used during deicing, the deicing fluids are difficult to dispose of once applied to the aircraft. As the deicing fluid is applied to the aircraft, most of the fluid is deposited onto the ground with the melting ice. Recycling of the fluid is an expensive process since it must be gathered, purified and again concentrated before it can be used. Disposal of the fluid requires that it be gathered and properly deposited in a hazardous toxic waste landfill. This is also an expensive and hazardous process.
In an attempt to alleviate the problems associated with using FPD fluids, it has been proposed that infrared radiation be applied to portions of the exterior surface of the aircraft to melt the ice thereon, the aircraft being exposed to ambient conditions. Although this process proposes some advantages over the use of FPD fluids, as discussed in the section entitled Brief Summary and Objects of the Invention, there are numerous limitations encountered with solely using infrared radiation to deice an aircraft exposed to the environment.